CAL_LID_L3_Tropospheric_APro_CloudySkyTransparent-Standard-V4-21 is the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Lidar Level 3 Tropospheric Aerosol Profiles, Cloudy Sky Transparent Data, Standard Version 4-20 data product. Data collection, which is ongoing, is collected using the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument. The CALIPSO lidar level 3 aerosol data product reports monthly mean profiles of aerosol optical properties on a uniform spatial grid. It is intended to be a tropospheric product, so data are only reported below altitudes of 12 km. All level 3 parameters are derived from the version 4.21 CALIOP level 2 aerosol profile product and have been quality screened before averaging. The primary quantities reported are vertical profiles of the aerosol extinction coefficient at 532 nm and its vertical integral, the aerosol optical depth (AOD). Aerosol type and spatial distribution information are also included. Averaged profile data is reported for all aerosols, regardless of type, and for mineral dust aerosols only. Classification of dust is based on the aerosol-type flags in the level 2 profile product. To keep level 3 file sizes manageable, there are four different types of level 3 files produced, depending on the sky condition and the temporal coverage of the data before averaging:

Description of the Four Sky Conditions (Day, Night)
1) All Sky: All level 2 columns are averaged, regardless of cloud occurrence,
2) Cloud-Free: Only cloud-free level 2 columns are averaged,
3) Cloudy-Sky, Transparent: Only level 2 columns containing transparent clouds are averaged, and
4) Cloud-Sky, Opaque: Only level 2 columns containing opaque clouds are averaged

CALIPSO was launched on April 28, 2006, to study the impact of clouds and aerosols on the Earth's radiation budget and climate. It flies in formation with five other satellites in the international "A-Train" (PDF) constellation for coincident Earth observations. The CALIPSO satellite comprises three instruments: the CALIOP, the Imaging Infrared Radiometer (IIR), and the Wide Field Camera (WFC). CALIPSO is a joint satellite mission between NASA and the French Agency CNES (Centre National d'Etudes Spatiales).